The present invention relates to apparatus and methods employing magnetic resonance apparatus in conjunction with other electrical appliances.
Copending, commonly assigned PCT International Publication WO98/52465, the disclosure of which is hereby incorporated by reference herein, describes an apparatus that incorporates magnetic resonance imaging capability in a relatively small device. Apparatus according to preferred embodiments disclosed in the ""465 publication includes a movable static field magnet adapted to apply a static magnetic field in a magnetic resonance volume at a predetermined disposition relative to the static field magnet, and may also include an energy applicator such as a high intensity focused ultrasound or xe2x80x9cHIFUxe2x80x9d device. The preferred apparatus, according to the ""465 publication, also includes positioning means for moving the static field magnet and the energy applicator to position the magnet and the applicator so that the magnetic resonance volume at least partially encompasses a region of the subject to be treated and so that the energy applicator is focused within the magnetic resonance volume. For example, the apparatus may include a chassis carrying both the static field magnet and the energy applicator, and the positioning means is arranged to move the chassis relative to the subject. The apparatus may further include ancillary equipment such as gradient coils mounted to the chassis or otherwise secured in position relative to the static field magnet for applying a magnetic field gradient within the magnetic resonance volume. Other ancillary equipment includes radio frequency equipment for applying radio frequency signals to the subject and receiving the resulting emitted radio frequency signals, hereafter referred to as magnetic resonance signals. Apparatus of this type may be used to acquire images of a region within the patient""s body, and may also be used to perform therapeutic procedures such as thermal ablation of tumors or other undesired tissues. The therapeutic procedures can be monitored using the magnetic resonance apparatus. Combined magnetic resonance imaging and hyperthermia procedures can also be performed using conventional magnetic resonance imaging apparatus having a large, fixed static field magnet defining a patient-receiving space. In this case, the patient is disposed inside the patient-receiving space of the magnet, and the HIFU unit is positioned in or near this space.
In magnetic resonance imaging procedures, the region of the subject to be imaged is subjected to a strong magnetic field using the static field magnet. Radio frequency signals are applied to the tissues of the subject within the imaging volume. Under these conditions, atomic nuclei are excited by the applied radio frequency signals and emit faint radio frequency signals, referred to herein as magnetic resonance signals. By applying appropriate gradients in the magnetic field during the procedure, the magnetic resonance signals can be obtained selectively from a limited region such as a two-dimensional slice of the subject""s tissue. The frequency and phase of the signals from different portions of the slice can be made to vary with position in the slice. By applying known techniques, it is possible to deconvolute the signals arising from different portions of the slice and to deduce certain properties of the tissues at each point within the slice from the signals. In some magnetic resonance procedures, these properties are determined for tissues at numerous closely-spaced locations so as to form an image of the subject, whereas in other magnetic resonance procedures, signals are acquired from one or more discrete locations within the subject, without developing an image of the subject.
A problem arises in procedures which employ an energy applicator such as a HIFU unit or other electrical apparatus in conjunction with the magnetic resonance procedures. Depending on its specific configuration, the electrical apparatus may emit unwanted radio waves, referred to herein as xe2x80x9cRF noisexe2x80x9d. For example, a HIFU unit, as disclosed in the copending application, may emit RF noise when it generates its bursts of high intensity focused ultrasound. The RF noise can interfere with the magnetic resonance signal received by the radio frequency equipment.
One aspect of the present invention provides methods of performing magnetic resonance procedures while also using an electrical appliance. As used in this disclosure, the term xe2x80x9celectrical appliancexe2x80x9d should be understood as referring to any device which is supplied with electrical energy or which emits RF noise, but does not include a part of the magnet and RF transmitter and receiver used to create the magnetic resonance image. Thus, the term xe2x80x9celectrical appliancexe2x80x9d includes, without limitation, devices such as energy applicators including HIFU units and other ultrasonic emitters, electronic instruments such as cathode ray tubes, television equipment such as the television cameras incorporated in certain endoscopes, and the like. According to this aspect of the invention, the electrical appliance is activated only when the RF receiver of the magnetic resonance apparatus is not actively receiving a magnetic resonance signal. Stated another way, the electrical appliance is activated only while the RF receiver is inactive.
As further described below, magnetic resonance procedures include cycles of timed steps, commonly referred to as xe2x80x9cpulse sequencesxe2x80x9d. One of these steps includes receiving the magnetic resonance signals emitted by the subject. The signals typically are received only during predetermined periods, referred to herein as xe2x80x9creceive intervalsxe2x80x9d. This aspect of the invention incorporates the realization that the receive intervals occupy only a fraction of the total cycle time, and that RF noise emitted during other parts of the cycle time will not adversely affect the magnetic resonance examination procedure to the same degree as RF noise emitted during the receive intervals. Preferred methods according to this aspect of the invention incorporate the further realization that many electrical appliances used in medical procedures can be operated intermittently with little loss of performance. In particular, operation of appliances such as HIFU units and other devices which are used to heat tissues can be interrupted during the receive intervals and still can provide satisfactory performance. Thus, in methods according to this aspect of the invention, an electrical appliance separate from the magnetic resonance system is deactivated during the receive intervals and is activated only during intervals when the RF receiver is not collecting magnetic resonance signals.
Methods according to a further aspect of the invention incorporate the realization that the RF noise emitted by an intermittently-operated electrical appliance can be tolerated if the data collected during particular pulse sequences adversely affected by such RF noise is discarded. Typically, the pulse sequence is repeated cyclically, and the magnitude of one or more of the gradients is changed in each cycle. The magnetic resonance data collected during the whole procedure is a composite of the information collected during the various cycles. For example, where the data is used to generate an image, the image is calculated from the data collected during all of the various cycles. In preferred methods according to this aspect of the invention, operation of the appliance is coordinated with the pulse sequence cycles so that cycles of the pulse sequence which are affected by such operation can be identified. For example, the appliance may be actuated at predetermined times, and the cycles of the pulse sequence may also be applied at predetermined times. Alternatively, the appliance may be actuated at times which have no preset relation to the times of particular pulse sequence, and actuation of the appliance may be detected so that the time of such actuation can be compared to the times of particular pulse sequence cycles to determine which cycles overlap with the appliance actuation. The RF receiver and/or data collection apparatus may be disabled during these cycles, so that no data is collected during these cycles and the magnetic resonance signals elicited in these cycles is discarded. Alternatively or additionally, magnetic resonance data associated with those cycles is discarded. Preferably, those cycles are repeated at a time when the appliance is inactive.
A further aspect of the invention provides apparatus which incorporates a magnetic resonance system having an RF receiver and an electrical appliance which is not part of the magnetic resonance system. The apparatus desirably includes means for disabling the electrical appliance while the RF receiver is active. As further discussed below, where the magnetic resonance system is controlled by a computer, the disabling means can include the control computer of the magnetic resonance system. Alternatively, the apparatus may include means for coordinating operation of the appliance with the times of pulse sequences so that pulse sequence cycles affected by operation of the appliance can be identified. In this case, the apparatus desirably also includes means for disabling the RF receiver or means for discarding data collected during the affected cycles. The apparatus may also include means for actuating the magnetic resonance system to repeat the affected cycles.